Document generated on 10/02/2021 11:33 a.m.

Atlantic Geology

A giant Rusophycus from the Arisaig Group (Siluro-Devonian) of Nova Scotia W. Tansathien and R. K. Pickerill

Volume 23, Number 2, August 1987 Article abstract A unique specimen of the trace fossil Rusophycus In the Moydart Formation URI: https://id.erudit.org/iderudit/ageo23_2art03 (Ludlow) of the Slluro-Devonlan Arisaig Group of northeastern Nova Scotia la at least 35 cm long, 18 en wide and 8.5 cm deep. The specimen represents the See table of contents largest ftusophycus yet described and can truly be regarded as a giant. It Is speculated that the most likely producer of tbe trace fossil was a homalonotld trlloblte. Publisher(s) Atlantic Geoscience Society

ISSN 0843-5561 (print) 1718-7885 (digital)

Explore this journal

Cite this article Tansathien, W. & Pickerill, R. K. (1987). A giant Rusophycus from the Arisaig Group (Siluro-Devonian) of Nova Scotia. Atlantic Geology, 23(2), 89–93.

All rights reserved © Atlantic Geology, 1987 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/

This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ A G I A N T RUSOPHYCUS FROM THE ARISAIG GROUP (SILURO-DEVONIAN) OF NOVA SCOTIA

y. Tansathien and R.K. P ic k e r ill Department of Geology, U niversity of New Brunswick Fredericton, New Brunswick, E3B 5A3

Date Received October 16, 1986 Date Accepted December 24, 1986

A unique specimen of the trace fossil Rusophycus in the Moydart Formation (Ludlow) of the Siluro-Devonian Arisaig Group of northeastern Nova Scotia is at least 35 cm long, 18 cm wide and 8.5 cm deep. The specimen represents the largest Rusophycus yet described and can truly be regarded as a giant. It is speculated that the most likely producer of the trace fossil was a homalonotid trilobite.

La Formation de Moydart (Ludlow) du Groupe siluro-dAvonien d'Arisaig, au nord-est de la Nouvelle-Ecosse, a livrA un exemplaire unique de l'ichnite Rusophycus mesurant au moins 35 cm an de long. 18 cm de large et 8.5 cm de profond. II s'agit 14 du plus grand Rusophycus jamais dAcrit et l'on est done en prAsence d'un vAritable gAant. Nous entrevoyons que 1'auteur de cette trace a pu etre un trilobite homalonotidA. [Traduit par le Journal]

INTRODUCTION is exposed in the Arisaig area of northwestern Nova Scotia on the southern shores of Northumberland The trace fossil Rusophycus (Hall, 1852) is an Strait (Fig. 1). The succession consists of 1400- ichnogenus commonly, if not universally, attributed 1500 m of black to greenish grey mudstone with to arthropods, in particular to trilobites (in coarse-grained siltstone and fine-grained sandstone marine environments) or isopods or notostracan interbeds. The group was subdivided by Boucot et branchiopods (in non-marine environments) and a l. (1974) into several formations and this study formed as a result of the producing organism is concerned with the Moydart Formation which resting, hunting or seeking protection (Osgood, occurs near the top of the succession. The Moydart 1970; Bergstrom, 1973). The trace is bilobate, the Formation is Ludlow (Upper Silurian) in age and has overall outline being elliptical, circular, rectan­ been subdivided by Boucot et a l. (1974) into a gular, heart- or v-shaped and tapering posteriorly lower green member, composed of green mudstone and (Alpert, 1976). The posteriorly tapering lobes siltstone with an increasing number of calcareous differentiate the ichnogenus from the morphologi­ interbeds toward the top, and an upper red member, cally similar traces Cardioichnus Smith and Crimes composed of red marls and concretionary limestones. and Chagrinichnites Feldman, Osgood, Szmuc and The specimen described herein was collected from Meinke. It ranges in age from Cambrian - Triassic the green member of the Moydart Formation at the and has been commonly reported from shallow marine base of a small cliff at Moydart Point, approxi­ and non-marine sequences throughout the world. mately 3.3 km southwest of Arisaig Harbour (Fig. (See Osgood, 1970; Osgood and Drennen, 1975). 1). Unfortunately the specimen was not found in The purpose of this short paper is to record an situ but was present in talus material located extremely large example of the ichnogenus recently approximately raid-way (stratigraphically) through discovered in the Siluro-Devonian Arisaig Group of the ca. 115 m+ of the green member exposed along Nova Scotia. Indeed, this example is, to our this section of shoreline (see Boucot et a l., knowledge, the largest Rusophycus yet recorded and 1974). can be truly regarded as a 'giant*. Despite brief reference to 'worm burrows' by Boucot et a l. DEP0SITI0NAL ENVIRONMENT (1974), the trace fossils Chondrites and Arthrophycus (- Muensteria) by Cant (1980) and Dineley (1963) studied the red member of the Chondrites, Skolithos, Helminthopsis, Planolites, Moydart Formation in some detail concluding that Palaeophycus, Gordia, Sealarituba, Teichichnus and the sequence was fluvial in origin. Boucot et a l. Linqulichnus by Pickerill and Hurst (1983) and (1974) and particularly Cant (1980) emphasized that Hurst and Pickerill (1986), detailed ichnological the remainder of the Arisaig Group was shallow studies of the sequence have not been attempted and marine in origin and formed on a storm-dominated certainly no previous recordings of Rusophycus have muddy shelf. Integrating both palaeontological and ever been made from within it. The specimen is sedimentological data, this conclusion has more currently housed in the Department of Geology, recently been reiterated by Pickerill and Hurst University of New Brunswick. (1983) and Hurst and Pickerill (1986) for the Llandovery Beechhill Cove and Ross Brook formations LOCATION AND STRATIGRAPHY of the Arisaig Group. The green member of the Moydart Formation contains intertidal strata The Siluro-Devonian Arisaig Group of Nova Scotia immediately below the contact with the red member

MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 23. 89-93 (1987)

0711-1150/87/020089-5$!.75/0 90 TANSATHIEN AND PICKERILL

Fig. 1. Location map of the Arisaig Group, northeastern Nova Scotia, illustrating distribution of the Moydart Formation and precise location where the Rusophycus described in this paper was collected (modified after Boucot et a/., 1974).

(Cant, 1980); the sequence is otherwise interpreted between some of the coarse ridges and extend out­ to be shallow subtidal in origin. This is further ward from the median axis parallel to the main reinforced by consideration of the total trace ridges for irregular distances of up to 2 cm. Each fossil assemblage within the formation, which of these is typically less than 1.5 mm in width and consists of Cruziana, Rusophycus, Skoiithos, poor-preservation does not allow conclusions on Helminthopsis, Chondrites, Palaeophycus and whether or not they are bunched. Planolites (the first three of which are typical members of the shallow subtidal Cruziana and REMARKS Skolithos ichnofacies of Seilacher (1964) and abundant and well-preserved shallow subtidal shelly Although Seilacher (1970, 1983) united under faunal communities (see Watkins and Boucot, 1975) Cruziana both long furrows (- Cruziana) and shorter dominated by brachiopods and bivalves. resting impressions (- Rusophycus) most subsequent authors, as we do herein, have preferred to retain DESCRIPTION the two as distinctive ichnogenera (see Crimes et a l., 1977 for a detailed review). One of the The specimen is preserved on the sole of an 8 cm essential differences between these two ichnogenera thick storm-related parallel- to low angle cross- is that in Cruziana there is evidence of forward laminated fine-grained sandstone as a large movement (Hofmann, 1979), thereby producing a positive feature (positive hyporelief) of broadly furrow or burrow (Goldring, 1985) whereas in convex outline (Fig. 2). Bioclastic material, in Rusophycus no such movement can be demonstrated. the form of disarticulated brachiopods and crinoid The specimen described herein does not indicate any columnals, is present in the basal 1-2 cm of the evidence of such movement and its overall outline sample, giving it an overall normally graded and convex morphology is typical of many previously appearance. The specimen is a posteriorly tapering described examples of Rusophycus. For these bilobate trace with a long elliptical outline (Fig. reasons we classify the trace as Rusophycus though 2). Maximum length is 35 cm but is incomplete; do admit it could possibly be regarded as a transi­ maximum width is 18 cm (9.5 cm right lobe, 8.5 cm tional form between Rusophycus and Cruziana partic­ left lobe) with a length: width ratio of approxi­ ularly as it lacks coxal, hypostome and podomere mately 2:1. Depth is variable and irregular with a impressions reported in some, but by no means all, maximum of 8.5 cm observed on the right lobe. Both examples of Rusophycus (e.g., Osgood, 1970; lobes possess prominent coarse ridges, each 0.2-1 Baldwin, 1977; Hofmann, 1979; Pickerill and cm in width and relief and oriented anterolaterally Fillion, 1984). The generally poor preservation of with a V-angle of 40-60° with respect to the median the specimen and lack of diagnostic morphological (groove) axis. There are 12 and 15 ridges on the indicators precludes ichnospecific assignment. right and left lobes respectively, the best pre­ As with other examples of both Cruziana and served occurring in the posterior portion of the Rusophycus (see Seilacher, 1970) it is tempting to specimen. Intervals between adjacent ridges range interpret the main coarse ridges on the specimen as from 1-2 cm on the right lobe and 1.2-2.3 cm on the having been formed by the endopodites and the left lobe. Delicately preserved fine ridges occur shorter, finer ridges as scratch marks dug by the MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 91

Fig. 2a. Rusophycus ichnosp. viewed from above (looking at the lower bedding surface). X 0.46. b. Lateral view of Rusophycus (upper portion of specimen as illustrated in a.) illustrating its broadly convex but irregular outline. X 0.46. 92 TANSATHIEN AND PICKERILL exites of the producing organism. In no way, 1978). Assuming, therefore, a trilobite origin for however, can this be established conclusively. As the Rusophycus described herein, what was the noted above, unfortunately the specimen length is likely culprit? It is of course dangerous to incomplete but by extrapolation is assessed to have speculate on producers of trace fossils when no been originally 40 cm. positive evidence is preserved. Nevertheless it is noteworthy that the Moydart Formation has only DISCUSSION AND CONCLUSIONS revealed two groups of trilobites, the Dalmanitidae and Homalonotidae. The low length; width ratio of Despite much palaeontological (e.g., McLearn, the body and generally low convexity of the former 1924; Harper, 1973; Peel, 1977), palaeoecological group is suggestive of a benthic mode of life (e.g., Levinton and Bambach, 1975; Pickerill and (Thomas and Lane, 1984) and the thoracic morphology Hurst, 1983; Hurst and Pickerill, 1986) and sedi- and smooth and wedge-shaped cephalon of the latter mentological (e.g., Lane and Jensen, 1975; Cant, group suggests adaptation to burrowing (Gill, 1980) research on the Arisaig Group, the ichnology 1949). Thus, either of these groups could have has been relatively overlooked. This is in spite been responsible for the production of the of the fact that the succession contains a rich and Rusophycus. It is notable, however, that varied ichnofaunal suite and that bioturbation is dalmanitid trilobites are numerically dominant in in all probability responsible for many of the offshore shelf facies (Thomas and Lane, 1984) presently observed sedimentary fabrics, particu­ whereas homalonotid trilobites prefer inshore larly in the mudstones. This report, even though coarse clastic facies (Thomas et a l., 1984). short, therefore represents the first formal Furthermore, homalonotid trilobites are consider­ account of at least one aspect of the ichnology. ably larger and in the Arisaig Group have been The distribution and systematics of the remaining recorded up to 10.5 cm in width (McLearn, 1924). trace fossils are the subject of ongoing study and It is tempting to suggest, therefore, that a will be published at a later date. homalonotid trilobite was in fact responsible for There have been several previous recordings of the production of the giant Rusophycus though this large examples of Rusophycus but none, to our conclusion must be regarded with caution. Notably, knowledge, as large as that described herein. however, Osgood and Drennen (1975) interpreted R. According to Seilacher (1970) Cruziana and bilobatum, an ichnospecies morphologically similar Rusophycus are most diversified and largest in to the specimen described herein, as having been Cambro-Ordovician strata and decrease in size and produced by homalonotids. diversity from the Silurian onwards. The majority of previous recordings of large Rusophycus would ACKNOWLEDGEMENTS tend to confirm this observation. Thus, for example, Hofmann (1979) has recorded Rusophycus We thank Rose Northrup who typed the manuscript carleyi from the Middle Ordovician Chazy Group 31 and Bob McCulloch and Paul Chenard for technical cm in length and 21 cm in width, and Draper (1980) assistance. Careful reviews of the preliminary has recorded forms resembling both R. dilata and R. manuscript by Guy Narbonne and Denis Fillion carleyi from the Early Ordovician Mithaka Formation greatly improved the content. Financial support of the Georgina Basin (Australia) up to 31 cm in for this and ongoing research on the Arisaig Group length. It is also noteworthy that Draper (1980) was provided from a CIDA No. 7282-84-1 scholarship reported several incomplete specimens with extrapo­ to the senior author and an NSERC Grant A 3857 to lated lengths of up to 36 cm. The previous largest the second author, both of which are gratefully Silurian recording is by Osgood (1970) who noted acknowledged. Rusophycus up to 25 cm in length from the Clinton Group in Cincinnati. The giant specimen of ALPERT, S.P. 1976. Trilobite and star-like trace fossils from Rusophycus documented herein suggests that caution the White-Inyo Mountains, California. Journal of Paleontology, must be exercised with respect to Seilacher's 50, pp. 266-240. BALDWIN, C.T. 1977. Rusophycus morgati: an asaphid produced (ibid.) conclusion on relative size of Rusophycus trace fossil from the Cambro-Ordovician of Brittany and in strata of different ages. This is further rein­ northwestern Spain. Journal of Paleontology, 51, pp. 411— forced when considering Shone's (1978) recording of 425. giant Cruziana, up to 23 cm in width, from the BERGSTROM, J. 1973. Organization, life and systematics of trilobites. Fossils and Strata, 2, pp. 1-69. Triassic Beaufort Group of South Africa. BOUCOT, A.J.. DEWEY, J.F., DINELEY, D.L., FLETCHER, R . , FYSON, Seilacher (1970) has summarized the arguments for W.K., GRIFFIN, J.G., HICKOX, C.F., MCKERROW, W.S. and and against a trilobite origin for marine examples ZIEGLER, A.M. 1974. Geology of the Arisaig Area, Antigonish of Cruziana and Rusophycus. While there is still County, Nova Scotia. Geological Society of America, Special Paper 139, 191 p. controversy as to whether trilobites were responsi­ CANT, D.J. 1980. Storm-dominated shallow marine sediments of ble for producing marine examples of Cruziana (see the Arisaig Group (Silurian-Devonian) of Nova Scotia. Whittington, 1980) it is universally accepted that Canadian Journal of Earth Sciences, 17, pp. 120-131. they were reponsible for producing most Rusophycus. CRIMES, T.P., LEGG, I., MARCOS, A. and ARBOLEYA, M. 1977. ?Late Precambrian-low Lower Cambrian trace fossils from Spain. In The discovery of trilobites preserved in situ Trace fossils 2. Edited by T.P. Crimes, and J.C. Harper. within Rusophycus (Osgood, 1970; Draper, 1980) Geological Journal, Special Issue 9, Seel House Press, together with the closely comparable morphological Liverpool, pp. 91—138. features preserved in some Rusophycus when compared DINELEY, D.L. 1963. The "Red Stratum" of the Silurian Arisaig Series, Nova Scotia. Canada Journal of Geology, 71, pp. 523- to the ventral morphology of trilobites leaves 524. little doubt that trilobites were responsible for DRAPER, J.J. 1980. Rusophycus (Early Ordovician ichnofossil) their production. Although most trilobites were from the Mithaka Formation, Georgina Basin. BMR Journal of small, some gigantic forms more than 20 cm in Australian Geology and Geophysics, 5, pp. 57-61. GILL, E.D. 1949. Paleozoology and taxonomy of some Australian length (Ural ichas attained a length of 70 cm) are homalonotid trilobites. Proceedings of the Royal Society of known from Cambrian to Devonian strata (Shone, Victoria, New South Wales, 61, pp. 61-73. MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 93

GOLDRING, R. 1985. The formation of the trace fossil Cruziana. morgati in Arenig strata of Bell Island, eastern Newfoundland. Geological Magazine, 122, pp. 65-72. Journal of Paleontology, 58, pp. 274-276. HALL, J. 1852. Palaeontology of New York 2. C. van Benthuysen, PICKERILL, R.K. and HURST, J.M. 1983. Sedimentary facies, Albany, 362 p. depositional environments and faunal associations of the HARPER, C.W. 1973. Brachiopods of the Arisaig Group (Silurian- Lower Llandovery (Silurian) Beechill Cove Formation, Arisaig, Lower Devonian) of Nova Scotia. Geological Survey of Canada Nova Scotia. Canadian Journal of Earth Sciences, 20, pp. Bulletin, 215, 163 p. 1761-1779. HOFMANN, H.J. 1979. Chazy (Middle Ordovician) trace fossils in SEILACHER, A. 1964. Biogenic sedimentary structures. In the Ottawa-St. Lawrence Lowlands. Geological Survey of Canada Approaches to Paleoecology. Edited by J. Imbrie and N.D. Bulletin, 321, pp. 27-59. Newell. John Wiley and Sons, New York, pp. 296-316. HURST, J.M. and PICKERILL, R.K. 1986. The relationship between SEILACHER, A. 1970. Cruziana stratigraphy of non-fossiliferous sedimentary facies and faunal associations in the Llandovery Palaeozoic sandstones. In Trace fossils. Edited by T.P. siliciclastic Ross Brook Formation, Arisaig, Nova Scotia. Crimes and J.C. Harper. Geological Journal, Special Issue 3. Canadian Journal of Earth Sciences, 23, pp. 705-726. Seel House Press, Liverpool, pp. 447-476. LANE, T.E. and JENSEN, L.R. 1975. Stratigraphy of the Arisaig SEILACHER, A. 1983. Upper Paleozoic trace fossils from the Gilf Group. In Ancient sediments of Nova Scotia. Edited by I. Kebir-Abu Ras area in southwestern Egypt. Journal of African Mck. Harris. Eastern Section, Society of Economic Earth Sciences, 1, pp. 21-34. Paleontologists and Mineralogists. Field trip guidebook, pp. SHONE, R.W. 1978. Giant Cruziana from the Beaufort Group. 75-96. Transactions of the Geological Society of South Africa, 81, LEVINTON, J.S. and BAMBACH, R.K. 1975. A comparative study of pp. 327-329. Silurian and recent deposit-feeding bivalve communities. THOMAS, A.T. and LANE, P.D. 1984. Autecology of Silurian Paleobiology, 1, pp. 95-124. trilobites. In Autecology of Silurian organisms. Edited by MCLEARN, F.H. 1924. Paleontology of the Silurian rocks of M.G. Bassett and J.D. Lawson. Special Papers in Arisaig, Nova Scotia. Geological Survey of Canada, Memoir Palaeontology, 32, pp. 55-69. 137, 180 p. THOMAS, A.T., OWEN, R.M., and RUSHT0N, A.W.A. 1984. Trilobites OSGOOD, R.G., JR. 1970. Trace fossils of the Cincinnati area. in British Stratigraphy. Special Report of the Geological Paleontographica Americana, 6, pp. 281-444. Society of London, 16, 72 p. OSGOOD, R.G., JR. and DRENNEN, W.T. III. 1975. Trilobite trace WATKINS, R. and B0UC0T, A.J. 1975. Evolution of Silurian fossils from the Clinton Group (Silurian) of east-central New brachiopod communities along the south-eastern coast of York State. Bulletin of American Paleontology, 287, pp. 299- Acadia. Geological Society of America Bulletin, 86, pp. 243- 348. 254. PEEL, J.S. 1977. Systematics and palaeoecology of the Silurian WHITTINGTON, H.B. 1980. Exoskeleton, moult stage, appendage gastropods of the Arisaig Group, Nova Scotia. Biologiske morphology, and habits of the Middle Cambrian trilobite Skrifter Kongelige Danske Videnskabemes Selskab, 21, 80 p. OJenoides serratus. Palaeontology, 23, pp. 171-204. PICKERILL, R.K. and FILLION, D. 1984. Occurrence of Rusophycus